Augmentation of human thymocyte proliferative responses by retinoic acid.

When 10(-5) to 10(-7) M all-trans retinoic acid (RA) was added to human thymocyte or tonsil lymphocyte cultures in the presence of mitogens or allogeneic stimulator cells, blastogenesis was increased up to 2.5-fold. No augmentation in proliferative responses of peripheral blood or spleen lymphocytes was observed. In the thymus, lymphocytes in the subclass of cells that did not bind peanut agglutinin (PNA) were responsible for the RA-induced enhancement. RA also increased the number of mitogen-stimulated thymocyte colonies developing in soft agar. These results indicate that the targets of RA activity must be lymphoid cells at a later stage of maturation than those identified by binding to PNA, and that one mechanism of RA enhancement is an increase in the number of lymphocytes that undergo blast transformation.